Concrete mixes are designed with the aid of test records obtained from field experience with the materials to be used. When field test results are not available, other means of mix proportioning can be used as described in this article. In any case, the proportions of ingredients must be selected to produce, so that for any three test specimens, the average strength equals or exceeds the specified compressive strength and no individual strength test (average of two specimens) falls Æ’c below by more than 500 psi. Æ’’c
The required average strength, Æ’cr depends on the standard deviation s expected.
Strength data for determining the standard deviation can be considered suitable if they represent either a group of at least 30 consecutive tests representing materials and conditions of control similar to those expected or the statistical average for two groups totaling 30 or more tests. The tests used to establish standard deviation should represent concrete produced to meet a specified strength within 1000 psi of that specified for the work proposed. For a single group of consecutive test results, the standard deviation is calculated
For an established supplier of concrete, it is very important to be able to document the value of s. This value is based on a statistical analysis in which Eq. (9.1)
is applied to at least 30 consecutive tests, and Eq. (9.2) is applied to two groups of consecutive tests totaling at least 30 tests. These tests must represent similar materials and conditions of control not stricter than those to be applied to the proposed project. The lower the value of s obtained from the tests, the closer the average strength is permitted to be to the specified strength. A supplier is thus furnished an economic incentive, lower cementitious materials content, to develop a record of good control (low s). A supplier who does maintain such a record can, in addition, avoid the expenses of trial batches.
When no such production record exists, the required average strength Æ’’cr , can cr be determined from Table 9.2. Documentation of the required average strength must be established. The documentation should consist of field strength records or trial mixtures confirming that the proposed concrete proportions will produce an average compressive strength equal to or greater than Æ’’cr . Alternatively, when an acceptable record of field test results is not available, the ACI 318 Building Code, with several restrictions, permits the use of trial batches as a basis for selecting initial proportions.
This condition is likely to occur when new sources of cement or aggregate are supplied to an established plant, to a new facility, such as a portable plant on the site, or for the first attempt at a specified strength more than 1000 psi above Æ’c previous specified strengths.
The ACI 318 Building Code includes provisions for proportioning concrete mixes based on other experience or information, if approved by the Engineer. This alternative procedure is restricted to proportioning concrete with a specified Æ’’c =< 4000 psi. The required average compressive strength Æ’’cr must be at least 1200 psi greater than Æ’. Concrete proportioned by this procedure must also conform to the c Codes durability requirements. These provisions are intended to allow the construction work to continue when there is an unexpected interruption in concrete supply and time does not permit tests and evaluation. These provisions are also aimed at small projects where the cost of trial batches is not justified.
The initially established proportions can be used during progress of a project only as long as the strength-test results justify them. The process of quality control of concrete for a project requires maintenance of a running average of strength-test results and changes in the proportions whenever the actual degree of control (standard deviation s) varies from that assumed for the initial proportioning. Equations (9.3) and (9.4) are applied for this analysis. With project specifications based on the ACI 318 Building Code, no minimum cementitious-materials content is required;
so good control during a long-time project is rewarded by permission to use a lower cementitious-materials content than would be permitted with inferior control.
Regardless of the method used for proportioning the basic initial proportions should be based on mixes with both air content and slump at the maximum permitted by the project specifications.
Other ACI 318 Building Code requirements for mix design are:
1. Concrete exposed to freezing and thawing or to deicing chemicals while wet should have air entrained within the limits in Table 9.3, and the water-cementitious materials ratio by weight should not exceed 0.45. If lightweight aggregate is used, should be at least 4500 psi. Æ’c
2. For watertight, normal-weight concrete, maximum water-cementitious materials ratios by weight are 0.50 for exposure to fresh water and 0.40 for seawater or deicing chemicals. With lightweight aggregate, minimum is 4000 psi for concrete Æ’’c exposed to fresh water and is 5000 psi for seawater or deicing chemicals. Æ’’c Although the Code does not distinguish between a concrete production facility with in-house control and an independent concrete laboratory control service, the distinction is important. Very large suppliers have in-house professional quality control. Most smaller suppliers do not. Where the records of one of the latter might indicate a large standard deviation, but an independent quality-control service is utilized, the standard deviation used to select Æ’’cr should be based on the proven record of the control agency. Ideally, the overdesign should be based, in these cases, on the record of the control agency operating in the concrete plant used.